1. Technical Field
The present invention relates to a technique for manufacturing a silicon substrate using continuous casting and, more particularly, to an apparatus for manufacturing a silicon substrate for solar cells using continuous casting, which can improve quality, productivity and energy conversion efficiency of the silicon substrate for solar cells, and a method for manufacturing a silicon substrate for solar cells using the same.
2. Description of the Related Art
Generally, a silicon substrate for solar cells is manufactured by solidifying molten silicon to prepare a single crystalline silicon ingot or polycrystalline silicon block, which in turn is subjected to a cutting process.
The single crystalline silicon ingot is manufactured from the molten silicon through crystal growth of a nucleus crystal by a Czochralski process.
The polycrystalline silicon block is manufactured through a unidirectional solidification process using a heat exchanger method (HEM) or Bridgman-Stockbarger method.
The prepared single crystalline silicon ingot or polycrystalline silicon block is subjected to several cutting processes to produce a silicon substrate.
FIG. 1 is a flow diagram of a conventional method of manufacturing a silicon substrate using a single crystalline silicon ingot.
Referring to FIG. 1, the single crystalline silicon ingot is subjected to the following processes to produce a silicon substrate.
First, cropping is performed to cut a shoulder and a tail of the ingot. Second, grinding is performed to grind the ingot to a desired size, that is, a desired diameter. Thirdly, flattening is performed to form a flat or notch portion on the ingot for recognition of a direction. Fourthly, slicing is performed to cut the ingot into a wafer shape. Fifthly, edge profiling is performed to process an edge of the wafer into a rounded shape to prevent breakage. Sixthly, lapping is performed to polish both sides of the wafer for removal of slicing defects, flatness enhancement and thickness adjustment. Seventhly, etching is performed to chemically remove a defect layer from the surface of the wafer. Last, polishing is performed to form a mirror surface(s) having excellent flatness on one or both sides of the wafer through a chemical or physical process.
In this case, about 50% of the ingot is removed as Kerf-loss through cropping, grinding, flatting, and slicing.
FIG. 2 is a flow diagram of a conventional method of manufacturing a silicon substrate using a polycrystalline silicon block.
Referring to FIG. 2, the polycrystalline silicon block is subjected to the following processes to produce a silicon substrate.
First, blocking is performed to cut the polycrystalline silicon block to a desired size. Second, cropping is performed to cut the head and tail of the silicon block. Thirdly, edge grinding is performed to process an edge of the silicon block into a rounded shape. Fourthly, slicing is performed to cut the silicon block into a wafer shape. Fifthly, polishing is performed to form a mirror surface(s) having excellent flatness on one or both sides of the wafer through a chemical or physical process.
Here, the silicon block undergoes 40% Kerf-loss through blocking, cropping, edge grounding, and slicing.
As such, when manufacturing the silicon substrate using the single crystalline silicon ingot or polycrystalline silicon block, 40% or more Kerf-loss occurs through several cutting processes. Thus, Kerf-loss causes an increase in manufacturing costs of the silicon substrate, which is a main element of a solar cell.
Meanwhile, in a recent process for manufacturing a silicon substrate for solar cells, a thin silicon substrate is directly obtained from molten silicon without the process of preparing the single crystalline silicon ingot or polycrystalline silicon ingot and the process of cutting the ingot or block, so that Kerf-loss can be fundamentally prevented in manufacture of the silicon substrate for solar cells.
In other words, this method allows the solidified silicon substrate to be directly manufactured from molten silicon. Thus, this method reduces manufacturing costs of the silicon substrate by up to 50% by eliminating the processes of preparing and cutting the ingot.
Currently, a technique for directly manufacturing a silicon substrate for solar cells can be classified into a vertical growth type and a horizontal growth type. The vertical growth type includes an edge-defined film-fed growth (EFG) process, a string ribbon (SR) process, and the like, and the horizontal growth type includes a ribbon growth on substrate (RGS) process, a silicon film process, a crystallization on dipped substrate (CDS) process, and the like.
FIG. 3 shows a schematic view of a solidification method in direct manufacture of a silicon substrate from molten silicon through the RGS process and microstructure of the silicon substrate manufactured by this method.
A technique for manufacturing a silicon substrate through the RGS process enables direct manufacture of the silicon substrate through horizontal growth. This technique secures a high production rate by rapidly removing latent heat from the manufactured silicon substrate through a lower substrate, providing higher productivity than any other techniques known in the art for direct manufacture of a silicon substrate for solar cells.
Referring to FIG. 3, when manufacturing a silicon substrate for solar cells using this technique, a solid/liquid interface is formed in the vertical direction and is at a right angle to the horizontal growth direction of the silicon substrate, thereby forming an inclined surface. As a result, the substrate has dense crystal grains and solidification of silicon proceeds along the inclined solid/liquid interface, so that impurities are segregated on the surface of the silicon substrate, causing quality deterioration of the silicon substrate.
When such a silicon substrate is used as a substrate for a solar cell, however, high energy conversion efficiency cannot be expected due to low quality of the substrate.
For the vertical growth technique, since the crystal growth direction is parallel to the proceeding direction, the crystal is grown in the longitudinal direction to have a large size, thereby improving energy conversion efficiency of the solar cell. However, despite such a merit, the vertical growth technique has a very low solidification rate, providing a disadvantage in terms of productivity.
On the other hand, for the horizontal growth technique, since the crystal growth direction is vertical to the proceeding direction, the crystal is grown in the thickness direction of the substrate to have a small size and undergoes precipitation of impurities, thereby causing lower energy conversion efficiency than the vertical growth technique. However, the horizontal growth technique can efficiently remove latent heat through the wide substrate, thereby enabling rapid growth of the substrate.
As such, such conventional methods of directly manufacturing a silicon substrate compromise productivity of the silicon substrate and energy conversion efficiency. Therefore, there is a need for a method for manufacturing a silicon substrate for solar cells, which can improve both productivity and quality of the silicon substrate.